Predictions - Will and Going to

We use will + infinitive without to when our prediction is based on intuition, knowledge, experience.

I think we will be able to have holidays on the moon in twenty years' time.

We use may/might + infinitive without to when we are not sure if something will happen or not.

I may go out tonight, if I'm not too tired.

We use be going to + infinitive when our prediction is based on evidence or facts that we can observe now.

Look at those clouds. It's going to rain.